Platinum group metals, e.g., platinum (Pt), palladium (Pd) and rhodium (Rh), are well-known precious metals playing an important role in many industrial applications such as jewellery and ornaments, electronics, telephone circuits, dental alloys, etc. Platinum and palladium have also long been used as reforming and hydrogenation catalysts in the petroleum or automobile industries, respectively. In the catalysts, small amounts of platinum or palladium are contained in large volumes of support materials, typically as a metal coat supported on alumina, zirconia or silica.
The petroleum or automobile industry catalysts containing platinum or palladium are usually discarded when its catalytic function deteriorates. Palladium content in the catalysis usually varies from 0.4 mass percent to 5.0 mass percent. Likewise, platinum, ruthenium and osmium content can vary from 0.1 mass percent to 0.5 mass percent. The spent catalysts may also contain 1.5-2.5 mass percent of rhenium and 3.0-5.0 mass percent of vanadium. Due to its low natural abundance, high price and increasing demands in various fields of industry, recovering and refining platinum group metals from various spent materials can be highly advantageous. Therefore, efficient recovery and purification of precious metals from spent catalysts has economic benefits.
Platinum and palladium in spent petroleum or automobile industry catalysts can be recovered through various hydrometallurgical or pyrometallurgical techniques. A hydrometallurgical treatment generally includes crushing (e.g., grinding and milling), leaching, filtration, and washing stages, which are followed by extraction of palladium from a solution (cementation or electrochemical process). The treatment may also include neutralization of the acid waste solution, filter cakes and acid off-gases.
For example, U.S. Pat. No. 6,455,018 describes a complex non-selective process for extraction of precious metals, e.g., platinum, palladium and rhodium from spent catalytic converters by grinding the catalysts, treating with sulphuric acid; calcination of the mixture between 150° C. and 450° C., leaching with chlorides and hydrochloric acid to obtain a solution of noble metals and any other metals along with a solid residue.
Basically, there are three process routes for hydrometallurgical treatment. The first route includes partial destruction of the alumina or zirconia catalyst support substrate and high-temperature chloritization with chlorine gas. In this case, palladium is completely transformed and the support substrate is partially transformed into solution or melt. Platinum or palladium can, for example, be extracted directly from a catalyst support substrate using leachates. As such, the catalyst can, for example, be leached in aqua regia or in hydrochloric acid with oxidants such as nitric acid, sodium chlorate, sodium hypochlorite and chlorine gas.
The second route includes full destruction of the alumina or zirconia catalyst support substrate. In this route, palladium and alumina are transformed into the solution or melt together. This route is based on total dissolution of the alumina substrate, thereby concentrating insoluble or sparingly soluble platinum and palladium into the residue.
The third route includes the processes of selected dissolution of palladium in solutions or in a metal collector with additional metal separation and palladium extraction without destruction of the alumina or zirconia catalyst substrate. In this case, palladium is completely transformed into solution or melt, whereas alumina remains in an insoluble form.
The disadvantages of conventional hydrometallurgical processes for recovery of platinum group metals from catalysts are associated with multistage processes, formation of large amounts of waste acidic or alkaline solutions. Likewise, these processes require a relatively great quantity of a metal-collector (e.g., Zn or Al) for extraction of the platinum group metals from the solution in the cementation stage of the process, usually in an amount of 7-8 times greater than the stoichiometric amount.
A conventional pyrometallurgical treatment of catalysts for recovery of platinum group metals generally includes a grinding (crushing and milling) stage and a smelting stage, followed by metal separation and palladium extraction from metal-collectors, for example, by an electrochemical process, along with slag treatment and neutralization of acid off-gases.
Precious metals within spent petroleum or automobile industry catalysts can also be recovered by chlorination at high temperature.
For example, U.S. Pat. No. 3,951,648 describes a process for recovering palladium from a spent catalyst having a palladium content of less than 5 percent. The method comprises putting the catalyst in contact with a gaseous chlorinated organic compound at a temperature sufficient for vaporizing the palladium and cooling the gaseous phase so as to recover the formed chlorinated palladium derivative.
U.S. Pat. No. 5,102,632 describes a two-stage method of recovering the noble metal content of a mixture of noble metals, e.g., platinum, palladium, and rhodium, from a spent catalyst. The method comprises first reductive chlorination at an elevated temperature by a gaseous chlorinating agent in the presence of a reducing agent (e.g., sulfur dioxide and carbon monoxide in stages). The method also comprises minimizing the amount of aluminum trichloride formed from either washcoat or underlying ceramic chlorination, and separating the aluminum trichloride or other washcoat chlorides from the products of chlorination of the noble metals, as by sublimation of the former in a reducing atmosphere at a temperature below the vaporization temperatures of the latter, thereby recovering the noble metal chlorides in a concentrated form. As a second stage, temperature is further increased in an atmosphere of chlorine alone, to volatilize rhodium trichloride for collection separately from the palladium and platinum chlorides volatilized previously in the first-stage reductive chlorination.
Despite prior art in the area of treatment of spent catalysts, there is still a need in the art for further improvement of a technique for recycling spent catalysts from the petrochemical, chemistry and automobile industries for recovery of platinum group metals from spent catalysts.
It would also be advantageous to have a method and apparatus for recovery of platinum group metals with low environmental impact agents able to extract palladium and/or other platinum group metals selectively and efficiently in mild conditions.
It would still be advantageous to have a method and apparatus for recovery of platinum group metals which can be easily industrialized and which would result in significant yield of the recovered metals.
The present disclosure satisfies the aforementioned need by providing a method for recovery of platinum group metals from a spent catalyst that includes one or more platinum group metals.